In medical microscopy the objective is to get three things, as good a resolution as possible, as much magnification as you can resolve, and the best depth of field as possible. These objectives in the design of optical systems generally require compromises. In a standard optical microscope the depth of field is inversely proportional to the magnification. As the magnification gets higher the depth of field gets narrower. If the object being viewed is sufficiently small that you have to magnify it to the point your eye can see it, the depth of field gets so narrow that there is no contour to what you are looking at and it blurs since there is no depth of field.
Another aspect in terms of microscopy is the fact that up until a few years ago blood was believed to be sterile. It was thought there was no fungus or bacteria in blood except under severe pathological conditions. Today, systemic micro-organisms in the blood have changed the early thinking that blood was sterile. The discovery of AIDS has resulted in a complete new understanding of the immune system of opportune organisms that take advantage of a depressed immune system. The standard research microscope cannot be used to study organisms because they are too small. In these microscopes the power of magnification is about 1000.times. or 1500.times.. Some sophisticated systems are available where you can get 2000.times., and 2500.times., but the depth of field gets so narrow they are only used for research. What is needed for examination of blood for its organisms is magnification in excess of 5000.times.. Also the depth of field must be sufficient that the total contour of the organisms can be seen. To give a general idea of the problem, if a person wants to see a red blood cell they need a depth of field of at least 7 microns. If the depth of field drops to 31/2 microns, it is only possible to see half the red blood cell. As the depth of field becomes progressively less, a person can only see a slice of the blood cell. If the interest being researched relates to membranes and things of this sort, a different approach has to be taken to break out of the limitations of a standard optical system.
Another important criteria that hadn't been looked at is most research microscopes concentrate on what is known as dark field microscopy. The problem is how do you illuminate and contrast what you want to look at? Also what type of light source could be used to try to enhance the contrast? Some of the new microorganisms need to be studied and are not seen in dark fields. It is therefore necessary to go to other types of optical modes, like phase contrasts, polarized light, as well as dark fields. This gets into things like differential interference phase. These are different ways of enhancing different types of light. As it turns out, it is necessary to do all of the three or four types of optical modes in addition to increasing magnification and depth of field. Due to the physics of optics, you can only get so much magnification if you want to maintain some sort of depth-of-field. It was determined that optimal magnification must be around 400.times. instead of 1000.times.. Therefore in order to gain the degree of required magnification necessary for visible analysis, it was determined that projection magnification could be used in conjunction with the initial optical magnification. A unique microscopy system has been designed by the applicants to combine a projection lens with the research microscope after it has received its optical magnification. The extra magnification produced by the projection lens does not affect the depth of field as the beam spreads out. The projected image may be received by a video camera and it is moved either toward or away from the projection lens in order to vary the amount of magnification. The resolution lens of the camera thus becomes a limitation. As better cameras and camera systems are developed, the resolution can be increased in the higher priced camera. It is important that the candle power required to illuminate the object has to be greater than what is presently used with research microscopes. The best microscopes on the market today have approximately 80 watts of illumination. This is clearly insufficient when we are talking about magnification levels of 10,000 or greater. It is therefore necessary that the light source be in the order of 150 watts of light so that there is in the order of 100 candle power available by the time the object is being viewed. When the T.V. camera is used it also requires so many candle power of lumens in order to get full color out of the camera.
In order to be able to use cheaper projection lenses, the system has been designed to only use the central flat portion of the lens which is substantially of the same quality for expensive lens and cheaper lenses.
It has been found that the basic system works quite well with a halogen white light 150 watts bulb. Since such a bulb would give off too much heat if positioned beneath the specimen platform, it has been necessary to locate the light source in a remote housing with its light being directed through a fiber optic cable whose exit end is positioned beneath the specimen platform. This is highly important because when it is desirable to look at live blood, extreme heat will destroy the organisms in the blood. Electron beam microscopes will go to 250,000.times. and they use an electron beam for resolution so that they have good resolution and high magnification. The problem is that you destroy the live blood specimen when it is under normal conditions on the specimen platform. A condenser lens is used at the front end of the fiber optic cable to focus the beam. This allows the system to be changed from one optical mode to another, such as, dark field, bright field, phase-contrast, single side band differential interference phase-contrast, and polarized and neutral density bright field.
The projection lens positioned in the bottom end of the projection tube allows the light to come out at about a thirty degree angle, but the only portion worried about is the center portion and the remainder is absorbed in the interior of the projection tube. Since the projected image is received on the camera lens the movement of the camera upwardly and downwardly in the projection tube will vary the magnification. When the camera is lowered to its lowest position, the magnification is that of the optical magnification system. When the camera is raised to its highest position, its magnification can go up as high as 12,000.times. with a 150 watts light source, but its depth of field remains unchanged between its lowest and highest positions since that has been determined entirely by the optical magnification, and not by the variable projection magnification. This opens a whole new window in medical microscopy because now small microorganisms can be seen with their alterations or destruction in the membrane integrity. The camera will produce whatever it sees in the tube housing on a video monitor at a size up to 40 times greater, depending on the size of the monitor. The depth of field on the video system remains constant.
The limitations and resolution relates to the light source that is used. The closer one comes to monochromatic or single wave length light the better the resolution will be. If higher magnification is required and better resolution necessary, it would simply be a matter of going to a laser type of light.